This invention relates, in general, to color video printer apparatus for producing a color photographic copy from a color video signal. More particularly, this invention relates to color video printer apparatus in which a color photographic copy is produced from a sequence of color field images which are normalized to have similar levels of black and white and in which the black and white levels of the exposed copy may be adjusted to the operator's preference.
Several techniques have been proposed for making a color copy from a color video signal. Most of these techniques have drawbacks. Where a still color camera is used to photograph an image displayed on a color monitor, a poor quality copy may result due to difficulties in positioning the camera relative to the monitor and in synchronizing the exposure time with the field or frame rate of the video signal. In another technique, the color video signal is divided into a sequence of red, green and blue signals which are sequentially displayed on a monochrome CRT. Color film is exposed to several frames of each color component image displayed on the CRT through stationary filters corresponding in color to the component signal. This technique is time consuming and not readily applicable to the copying of a moving video scene. Other video printing techniques are disadvantageous due to physical size, due to structural and circuit complexity, due to high cost and/or due to the necessity of replenishing toxic chemicals or handling messy thermal transfer material. Although in many of these techniques, it is possible to adjust the image characteristics of the final copy, such adjustments have been effected by expensive or complex controls using digital processing circuitry or the like.
In commonly-assigned, copending U.S. application Ser. No. 776,232, entitled COLOR VIDEO PRINTER, there is disclosed a color video printer which obviates those disadvantages by providing a compact, economical and easy to use color video printer which produces a full resolution color copy of a color video signal with short exposure and processing time. As disclosed, the color video printer includes a monochrome display device and a color filter having first, second and third primary color filters sequentially movable into an optical path between the display device and a self-processing photographic element positioned at an exposure station. A video signal circuit applies a sequence of primary color component signals to the display device as corresponding color filters move into the optical path to expose the element to a sequence of six color field images constituting a full resolution color image. A composite color video signal having luminance and chrominance components is demodulated into three concurrent primary color component signals from which one component signal is selected by a sequential gate for application to the display device.
In such a color video printer, it is desirable that the printer operator be able to adjust the black and white levels in a finished print to suit individual aesthetic tastes by using a simple and inexpensive control technique. At the same time, since the copy is produced from a sequence of field images, it is also desirable that the black and white levels of each field be normalized with respect to each other to effect consistent image characteristics from field to field.